Method of hardening steel



f?atenteei July 2d, 193% stares lli filfliid METHOD OF HABDENENG STEEL Vincent '1. Malcolm, Springfield, Mass", assignor to Chapman Valve Manufacturing Company,

Indian @rchard, Mass,

Massachusetts a corporation of No Drawing. Application November 2, 1931, Serial No. 572,728

- 2 Claims.

- This invention relates to improvements in steel manufacture and is directed more particularly to improvements in the method of providing steel with hardened surfaces and the product-thereof.

The principal objects of the invention are directed to the provision of improvements in methods for producing steel with hardened surfaces.

According to special features of the invention steel of novel composition is treated in a novel way to bring about the desired hardening of the surfaces thereof.

According to one novel feature of the invention I employ a steel of novel composition which readily lends itself to a surface hardening operation as by being subjected to heat when immersed in a bath. By reason of the novel action of the heat and bath on the novel steel, it is possible to provide surfaces which are very hard and of considerable depth and which are not brittle. This according to the invention is accomplished in a short space of time without the employment of.

expensive apparatus, all to the end that there is provided a surface hardened steel of low cost.

In the well-*lmown nitriding operations, steel is enclosed in a container and subjected to the action of ammonia gas at temperatures ranging from 850 degrees to 1200 degrees Fair. Not only is expensive'equipment required for'such operations but the hardening operation requires from 40 to 90 hours to obtain the desired surface hardening which varies in thickness from .020 to .030 inches.

The nitriding operation and the apparatus required for the treatment of the steel necessarily results in nitrided steel being very costly. It is to overcome this objection that the present invention is directed, while at the same time it is possible to produce a steel which is equal if not superior in all its important respects to the steel produced by the nitriding processes. Not only does the invention provide the hardened surfaces in a novel way but the steel by reason of its novel composition has many desirable qualities.

According to this invention I employ a steel alloy which contains as essential constituents, chromium and tungsten. Depending somewhat upon the particular results to be obtained the steel will consist of the following elements within I the range of proportions stated.

Steel of the foregoing alloy may be fabricated or formed into objects of various shapes and sizes such as castings, forgings, tubes, rods, bars, sheets and the like.

According to the invention the steel is immersed in a bath of salts which is heated to a temperature within a range from 1400 degrees to 1750 degrees Fah. or thereabouts.

The length of time of immersion of the steel in the bath may vary and this will of course depend largely on the size of the work being treated. For instance, a rod of steel one inch in diameter and 12 inches long may be suitably heated and treated during a 20-minute period. Pieces of larger dimensions may require a somewhat longer immersion, while of course smaller pieces may require some less time for the surface hardening operation.

The salts employed may onsist of potassium carbonate, sodium carbonate, barium carbonate, potassium cyanide, and sodium cyanide. All of these will function for the purposes desired and various combinations and proportions of them may be used.

It has been discovered that when steel of the alloy specified is immersed in a bath of salt and heated within the temperature range mentioned, that the steel will have a hardened surface which hardening is fully as deep if not deeper than that obtained by the nitriding processes with nitriding steels. At the same time the hardened surface is decidedly less brittle than that obtained by the nitriding or similar processes.

It has been shown that the surface hardness ranges from 800 to 1000 Brinell hardness as determined by the well-known Monotrone apparatus.

According to the invention and previous to the immersion of the steel in the salt bath, it is desirable to heat treat the steel by subjecting it to elevated temperatures ranging from 1475 to 1750 degrees Fah. The steel is then normalized or quenched in oil, subsequent to which it is preferably' drawn at temperatures ranging from 300 to 1300 degrees Fah. depending upon the physical properties desired in the finished steel.

By means of the method described, it is possible to produce a steel which not only has a central portion or core of extreme tensile strength and toughness but which has a hardened surface of great depth and toughness and which also is not brittle as compared with steels of other analysis which are surface hardened in other ways. Following the method described, it is possible in a comparatively short space of time to provide the novel steel with hardened surfaces and thereby eifect economy in the manufacturing costs and at the same time provide steel which is far superior in all its important respects to any heretofore known. I

The steel of the analysis mentioned has very desirable, qualities so that by means of the invention it is possible to make use of the qualities and at the same time take advantage of the surface hardening efiect.

Various changes and modifications may be made in the practise of the invention without departing from the spirit and scope thereof. Therefore what it is desired to claim and secure by Letters Patent of the United States is:

1. The method of making steel having a hard core and hardened surface which consists in, heat-treating a steel alloy to provide a hard core by subjecting it to a temperature within a range of from 1400 to 1750 degrees F., said alloy consisting of approximately 2 to 8 per cent of chromium, approximately .3 to 2 percent of silicon, approximately .75 to 2 percent of tungsten, approximately .15 to .5 percent of carbon, approximately .4 to .6 percent of manganese, approximately .05 percent of phosphorus and of sulphur, and the balance iron, quenching the said heat-treated alloy in oil and drawing it within a temperature range of from 300 to 1300 degrees F., and then immersingthe alloy in a salt bath containing a carbonate and a cyanide, said bath being heated within a range of from 1400 to 1750 degrees F., whereby the surface of said alloy is hardened.

2. The method of making steel having a hard core and hardened surface which consists in, heat treating a steel alloy to provide a hard core by subjecting it to a temperature within a range of from 1400 to 1750 degrees F., said alloy consisting of chromium from 2 percent to 8 percent, silicon from .3 percent to 2 percent, tungsten .75 percent to 2 percent, carbon from .15 percent to .5 percent, manganese from .4 percent to .6 percent, phosphorus approximately .05 percent, sulphur approximately .05 percent, and the balance iron, quenching the heated alloy in oil, drawing the heated alloy within a temperature range of from 300 to 1300 degrees F., and then in immersing said heat treated alloy in a cyanide salt bath heated within a range of from 1400 to 1750 degrees F., whereby the surface of said alloy is hardened.

VINCENT T. MALCOLM. 

